The All-Vanadium Redox Flow Battery, referred to here as the V/VRB is described in the following patents: Australian patent 575247, AU 696452, AU 704534, U.S. Pat. Nos. 6,143,443 and 6,562,514, while the Vanadium Bromide Redox Flow cell, referred to here as the V/BrRB is described in PCT/AU02/01157, PCT/GB2003/001757 and PCT/AU2004/000310. Both batteries employ a vanadium electrolyte solution in both half-cells, but in the case of the V/VRB, a vanadium sulphate solution is used in both half-cells and the cell employs the V(II)/V(III) couple in the negative half-cell and a V(IV)/V(V) couple in the positive half-cell electrolyte. The Vanadium Bromide Redox Battery (V/BrRB) employs a vanadium bromide electrolyte solution in both half-cells and the cell employs the V(II)/V(III) couple in the negative half-cell and a Br−/Br3− or Br−/ClBr2− couple in the positive half-cell electrolyte. The positive half-cell couples are also referred to as halide/polyhalide couples. The highly oxidising V(V) or polyhalide ions in the charged positive half-cell solutions lead to rapid deterioration of most polymeric membrane materials, so only limited types of membranes can be employed for long life. The membrane it can be postulated is therefore the most important component of the Vanadium Redox Batteries (VRBs) and a great amount of effort has been put into the selection or development of a suitable membrane that can offer the following characteristics: good chemical stability in the acidic vanadium sulphate or vanadium bromide electrolytes, good resistance to the highly oxidising V(V) or polyhalide ions in the charged positive half-cell electrolyte, low electrical resistance, low permeability to the vanadium ions or polyhalide ions, high permeability to the charge-carrying hydrogen ions, good mechanical properties and low cost. To date, only limited membranes have been shown to possess all or most of these characteristics. The perfluorinated membranes such as Gore Select, Nafion 112, Nafion 115 and Nafion 117 have been used with some success in the Vanadium Sulphate Electrolyte V/VRB, but these have tended to show blistering or fouling and excessive water transfer behaviour during cycling. The degree of water transfer in the Vanadium Bromide Electrolyte Cell can be so high when these membranes are employed, that after only a small number of cycles, the capacity and coulombic efficiency drops dramatically. The high level of water or solution transfer is caused by the high level of swelling of the extruded Nafion membranes in water and in the acidic vanadium sulphate and vanadium bromide electrolytes that increases the pore size and therefore the transfer of water and vanadium or polyhalide ions across the membrane. The degree of swelling is a function of the ionic strength of the solution, but is most severe in distilled water. When a Nafion membrane is transferred from water to the vanadium electrolyte, considerable shrinkage can occur due to the differences in ionic strength between the pores and the external solution. This means that the Nafion membranes must first be equilibrated in the battery electrolytes prior to stack assembly since any shrinkage after assembly could give rise to ripping of the membrane sheets. Similarly Nafion membranes cannot be assembled dry since the high level of swelling, up to 10%, that occurs when subsequently immersed in the V/VRB and V/BrRB electrolytes, will lead to creasing and possible damage of the membrane in the cell stack. Furthermore, once wet, Nafion membranes should not be allowed to dry out as this could cause cracking of the resin and irreversible damage to the membrane. All of these issues give rise to considerable problems in the handling, storage, assembly and operation of redox flow batteries using Nafion membranes. Nafion membrane are also subject to fouling, so require higher purity electrolytes that significantly adds to the cost of the V-VRB and V/BrRB. These factors, combined with the high electrolyte volume transfer during charge-discharge cycling, has limited the performance of the Nafion membranes in the V/VRB and V/BrRB and combined with the high cost, has restricted its practical use to date. Gore Select membranes have also been tested in the Vanadium Redox Batteries, but significant blistering of the membrane was observed in both the vanadium sulphate and vanadium electrolytes after several weeks of cycling. This showed that the Gore Select perfluorinated membranes are unsuitable for use in the Vanadium Redox Batteries.
Polysulphone membranes have also shown good chemical stability and good performance in the Vanadium Sulphate Electrolyte V/VRB, but have also been susceptible to fouling and loss of performance, requiring very high purity vanadium that adds to the cost of the electrolyte. In particular the presence of trace amounts of silica in the vanadium electrolyte can cause serious fouling, so that costly purification processes are needed to produce vanadium oxides with low silica levels. A further difficulty with this membrane is the need to keep the membrane wet at all times. If allowed to dry out, the membrane can crack and become damaged, but equally serious is the fact that many of the commercial membranes based on polysulphone, become hydrophobic on drying and require a long and sometimes difficult process to restore their hydrophilicity. Despite these problems, polysulphone membranes have been used successfully in the V/VRB, but their performance in the Vanadium Bromide Redox Cell has been very poor. This is due to their anion exchange properties that allow the polyhalide ions to pass through unimpeded, leading to rapid self discharge. To date, therefore, no single membrane has been found to perform well in both the V/VRB and V/BrRB.
It is an object of the present invention to address or ameliorate one or more of the abovementioned disadvantages or at least provide a useful alternative.
Any discussion of documents, publications, acts, devices, substances, articles, materials or the like which is included in the present specification has been done so for the sole purpose so as to provide a contextual basis for the present invention. Any such discussions are not to be understood as admission of subject matter which forms the prior art base, or any part of the common general knowledge of the relevant technical field in relation to the technical field of the present invention to which it extended at the priority date or dates of the present invention.